Add a ci test for cabal-install self-upgrade policy

[23Skidoo]

In 6c2eba7fca96872b6c8e3bde711feadf3491427f we clarified our cabal-install support policy with respect to self-upgrade: upgrading to the latest version with cabal install cabal-install must work with all cabal-install versions released in the last three years. As discussed in #4250, we should also have a CI check that this guarantee actually holds. Testing that cabal install Cabal/ cabal-install/ succeeds with cabal-install 1.18 should be enough.